1. Field of the Invention
This invention relates to a method of reducing the viscosity of a fracturing fluid utilized to stimulate a subterranean formation by incorporation of a catalyst or activator for the breaker system.
2. Description of the Prior Art
In the production of hydrocarbons or other fluids from subterranean rock formations penetrated by wellbores, a commonly used technique for stimulating such production is to create and extend fractures in the formation. Most often, the fractures are created by applying hydraulic pressure to the subterranean formation from the wellbores penetrating them. That is, a fracturing fluid is pumped through the wellbore and into a formation to be fractured at a rate such that the resultant hydraulic pressure exerted on the formation causes one or more fractures to be created therein. The fractures are extended by continued pumping of fluid which may contain additives such as sand or other insoluble particulate proppant material to facilitate flow of hydrocarbons through the fractures to the wellbore. To facilitate the fracturing process and transport of the propping agent into the created fracture, the fracturing fluid normally is thickened, viscosified, or gelled by incorporation of a gelling agent in the fluid. After completion of the fracturing process, in order to effect clean-up of the gelled fluid, a breaker additive or system is typically employed which helps lower significantly the viscosity of the fracturing fluid thereby permitting or facilitating return of the fluid to the wellbore for cleaning the fracture passageway for the production of hydrocarbons.
Fracturing fluid systems comprising aqueous solutions of polysaccharide compounds are well known in the art. One particularly desirable system for lower temperature formations, that is formations having a temperature below about 140.degree. F., is an aqueous solution of a polysaccharide that is crosslinked with a borate ion source although other transition metals also may be utilized to crosslink the polysaccharide. Of particular interest are borate crosslinked systems employing galactomanan gums such as guar or modified guars such as hydroxypropylguar.
One known breaker system for polysaccharides employs ammonium or alkaline metal persulfates to break the gel. However, in relatively low temperature formations a breaker aid such as disclosed in U.S. Pat. Nos. 4,250,044 or 4,969,526 comprising a tertiary triethanolamine compound has been utilized to accelerate the rate of breaking of the gel.
It would be desirable to provide alternate methods by which the ammonium or alkaline metal persulfates can be catalyzed or activated to break a polysaccharide gel at temperatures below about 140.degree. F.